1. Technical Field
This invention is generally related to vessels and processes useful in an oxidative coupling of methane (“OCM”) reaction.
2. Description of the Related Art
Alkene hydrocarbons are also referred to as ‘olefins’ within the petrochemical industry and can include any unsaturated hydrocarbon compound containing at least one carbon-to-carbon double bond. Alkenes are used widely within the chemical industry for their general reactivity and ability to polymerize or oligomerize into longer chain hydrocarbon products such as synthetic fuels. Although alkenes are naturally occurring their demand far exceeds the natural supply. Consequently, the vast majority of alkenes are produced via thermal or catalytic cracking of longer chain mixed hydrocarbons such as crude oil or light ends such as napthas. An increasing worldwide demand for longer chain hydrocarbon oils, lubricants and fuels places a demand on hydrocarbon cracking operations to optimize or maximize the formation of longer chain hydrocarbons requiring minimal post-cracking processing to provide such high-demand products. As such, the production of shorter chain alkenes, such as ethylene (IUPAC designation “ethene”) using steam or catalytic cracking is in economic tension with the production of generally more valuable longer chain hydrocarbons. Shorter chain alkenes are typically produced using gaseous or liquid light hydrocarbons which are steam cracked at temperatures of 750° C. to 950° C. The cracked gas contains multiple alkene hydrocarbons, including ethylene, and is immediately quenched to halt the numerous secondary (olefin-consuming) free radical reactions within the off-gas. The various alkenes can then be separated from the remaining quenched cracked gas via distillation.
Natural gas is a naturally occurring mixture of hydrocarbon gases including methane and containing up to about twenty percent concentration of higher hydrocarbons such as ethane and small quantities of impurities such as carbon dioxide and hydrogen sulfide. With hundreds of years and trillions of cubic feet of proven, unextracted, natural gas reserves, natural gas potentially provides a rich source of hydrocarbons. Unfortunately, natural gas, or more specifically the methane found in natural gas is expensive to transport for extended distances except by pipeline. Even with the use of pipelines, methane requires significant capital investment in the pipeline itself and incurs significant operational expense in the recompression stations needed to maintain a reasonable pipeline flow. However, restricting transport to pipelines essentially relegates such methane sources to the role of a regional supply, meaning that unless a local demand exists for the methane, the natural gas supply is “stranded”—available for extraction but without a local demand making the extraction economically attractive and practical.
Historically methane has been converted to longer chain hydrocarbons through steam reforming to provide a synthesis gas (“syn-gas”), containing a mixture of carbon monoxide and hydrogen, which is then used as a feedstock to a Fischer-Tropsch process which converts the carbon monoxide and hydrogen into liquid hydrocarbons (often referred to as a “gas-to-liquids” or “GTL” process) that include synthetic lubrication oils and synthetic fuels. While periodically used on a widespread basis, for example by Germany during World War II, the popularity of the Fischer-Tropsch process is hampered by high capital costs associated with the construction of the process, and the high operation and maintenance costs associated with the ongoing operation of the process. However, even with Fischer-Tropsch, the ability to convert methane to short chain alkenes such as ethylene is extremely limited.
Ethylene is widely used in chemical industry, and historically the worldwide production of ethylene has exceeded that of any other organic compound. Ethylene is used in a wide variety of industrial reactions, including: polymerization, oxidation, halogenation and hydrohalogenation, alkylation, hydration, oligomerization, and hydroformylation. Within the United States and Europe, approximately 30% of the ethylene produced is used in the manufacture of three chemical compounds—ethylene oxide which is used as a precursor in the production of ethylene glycol; ethylene dichloride which is used as a precursor in the production of polyvinylchloride; and ethylbenzene which is used as an intermediate in the production of styrene and polystyrene. Significant quantities of ethylene (approx. 60% of total use) are consumed in the production of various forms of polymerized ethylene, or “polyethylene.”
The oxidative coupling of methane (“OCM”) reaction promotes the formation of alkene hydrocarbons such as ethylene using an exothermic reaction of methane and oxygen over one or more catalysts according to the following equation:2CH4+O2C2H4+2H2OThe reaction is exothermic (ΔH=−67 kcal/mole) and historically was conducted at very high temperatures of from about 750° C. to about 950° C. to provide a C2 (ethane+ethylene) yield reported to be in the range of 15%-25%. Despite intensive efforts to develop catalysts for the OCM process over the last 30 years, there exists a need for an economic and reliable direct conversion of methane to higher molecular weight hydrocarbons.
The value associated with ethylene production is significant, estimated in excess of $150 billion (US) per year. Used as an intermediate and a raw material feedstock throughout the petrochemical industry, the current ethylene production process via steam cracking consumes greater quantities of energy than nearly all other commodity chemical processes, consumes valuable fractions recovered from crude oil, and is one of the largest contributors to global greenhouse gas (“GHG”) emissions in the chemical industry.